17 the Respiratory System

Mechanics of Breathing The Respiratory System 17 Bones and Muscles of the Thorax Surround the Lungs Pleural Sacs Enclose the Lungs Airways Connect Lungs to the External Environment The Airways Warm, Humidify, and Filter Inspired Air Alveoli Are the Site of Gas Exchange Pulmonary Circulation Is High-Flow, Low-Pressure G a s L a w s Air Is a Mixture of Gases Gases Move Down Pressure Gradients Boyle’s Law Describes Pressure-Volume Relationships Ventilation Lung Volumes Change During Ventilation During Ventilation, Air Flows Because of Pressure Gradients Inspiration Occurs When Alveolar Pressure Decreases Expiration Occurs When Alveolar Pressure Increases Intrapleural Pressure Changes During Ventilation Lung Compliance and Elastance May Change in Disease States Surfactant Decreases the Work of Breathing Airway Diameter Determines Airway Resistance Rate and Depth of Breathing Determine the Effi ciency of Breathing Gas Composition in the Alveoli Varies Little During Normal Breathing Ventilation and Alveolar Blood Flow Are Matched Auscultation and Spirometry Assess Pulmonary Function This being of mine, whatever it really is, consists of a little fl esh, a little breath, and the part which governs. — Marcus Aurelius Antoninus ( C . E . 121–180) Background Basics Ciliated and exchange epithelia Pressure, volume, fl ow, and resistance Pulmonary circulation Surface tension Colored x-ray of the lung Autonomic and somatic showing the motor neurons branching Velocity of fl ow airways. 600 Mechanics of Breathing magine covering the playing surface of a racquetball court cavity to control their contact with the outside air. Internalization (about 75 m2 ) with thin plastic wrap, then crumpling up the creates a humid environment for the exchange of gases with the wrap and stuffi ng it into a 3-liter soft drink bottle. Impossible? blood and protects the delicate exchange surface from damage. I Maybe so, if you use plastic wrap and a drink bottle. But the lungs Internalized lungs create another challenge, however: how to of a 70-kg man have a gas exchange surface the size of that plastic move air between the atmosphere and an exchange surface deep wrap, compressed into a volume that is less than that of the bottle. within the body. Air flow requires a muscular pump to create Th is tremendous surface area for gas exchange is needed to supply pressure gradients. More complex respiratory systems therefore the trillions of cells in the body with adequate amounts of oxygen. consist of two separate components: a muscle-driven pump and a Aerobic metabolism in cells depends on a steady supply thin, moist exchange surface. In humans, the pump is the muscu- of oxygen and nutrients from the environment, coupled with loskeletal structure of the thorax. Th e lungs themselves consist of the removal of carbon dioxide. In very small aquatic animals, the exchange epithelium and associated blood vessels. simple diff usion across the body surface meets these needs. Dis- Th e four primary functions of the respiratory system are: tance limits diff usion rate, however, so most multicelled animals 1 Exchange of gases between the atmosphere and the blood. require specialized respiratory organs associated with a circula- Th e body brings in O for distribution to the tissues and tory system. Respiratory organs take a variety of forms, but all 2 eliminates CO waste produced by metabolism. possess a large surface area compressed into a small space. 2 2 Homeostatic regulation of body pH. The lungs can alter Besides needing a large exchange surface, humans and 17 body pH by selectively retaining or excreting CO . other terrestrial animals face an additional physiological chal- 2 3 Protection from inhaled pathogens and irritating substances. lenge: dehydration. The exchange surface must be thin and Like all other epithelia that contact the external environ- moist to allow gases to pass from air into solution, and yet at ment, the respiratory epithelium is well supplied with de- the same time it must be protected from drying out as a result fense mechanisms to trap and destroy potentially harmful of exposure to air. Some terrestrial animals, such as the slug substances before they can enter the body. (a shell-less snail), meet the challenge of dehydration with be- 4 Vocalization. Air moving across the vocal cords creates havioral adaptations that restrict them to humid environments vibrations used for speech, singing, and other forms of and nighttime activities. communication. A more common solution is anatomical: an internalized respiratory epithelium. Human lungs are enclosed in the chest In addition to serving these functions, the respiratory system is also a signifi cant source of water loss and heat loss from the body. These losses must be balanced using homeostatic RUNNING PROBLEM compensations. In this chapter you will learn how the respiratory system Emphysema carries out these functions by exchanging air between the environment and the interior air spaces of the lungs. Th is exchange You could hear her whistling, wheezing breathing preceding her down the hall. “Diagnosis: COPD,” reads Edna Wilson’s is the bulk fl ow of air, and it follows many of the same principles patient chart. COPD—chronic obstructive pulmonary that govern the bulk fl ow of blood through the cardiovascular disease—is the name given to diseases in which air system: exchange is impaired by narrowing of the lower airways. 1 Flow takes place from regions of higher pressure to regions Most people with COPD have emphysema or chronic of lower pressure. bronchitis or a combination of the two. Individuals in whom 2 A muscular pump creates pressure gradients. chronic bronchitis predominates are sometimes called “blue bloaters,” owing to the bluish tinge of their skin (from low 3 Resistance to air fl ow is infl uenced primarily by the diam- blood oxygen levels) and a tendency to be overweight. In eter of the tubes through which the air is fl owing. contrast, patients with emphysema have been nicknamed Air and blood are both fl uids. Th e primary diff erence be- “pink puff ers.” They tend to be thin, have normal (pink) skin tween air fl ow in the respiratory system and blood fl ow in the coloration, and often breathe out through pursed lips, which circulatory system is that air is a less viscous, compressible mix- helps open their airways. More than 12 million people in ture of gases while blood is a noncompressible liquid. the United States have COPD. Its most common cause is smoking, and most people can avoid the disease simply by not smoking. Unfortunately, Edna has been a heavy smoker for 35 of her 47 years. The Respiratory System The word respiration has several meanings in physiology ( Fig. 17.1 ). Cellular respiration refers to the intracellular reaction of oxygen with organic molecules to produce carbon 601 Mechanics of Breathing consists of structures involved in ventilation and gas exchange EXTERNAL RESPIRATION ( Fig. 17.2 ): The respiratory and circulatory systems coordinate to move oxygen 1 Th e conducting system of passages, or airways , that lead and CO between the atmosphere and the cells. 2 from the external environment to the exchange surface of Exchange I: CO2 O2 the lungs. atmosphere 2 Th e alveoli (singular alveolus ) {alveus, a concave vessel}, to lung a series of interconnected sacs and their associated pulmo- (ventilation) Airways nary capillaries. Th ese structures form the exchange surface, where oxygen moves from inhaled air to the blood, Alveoli of lungs and carbon dioxide moves from the blood to air that is CO2 O2 about to be exhaled. Exchange II: lung to blood 3 Th e bones and muscles of the thorax (chest cavity) and ab- O CO2 2 domen that assist in ventilation. Transport of Pulmonary gases in Th e respiratory system can be divided into two parts. Th e circulation the blood upper respiratory tract consists of the mouth, nasal cavity, pharynx, and larynx. Th e lower respiratory tract consists of the trachea, two primary bronchi { bronchos , windpipe; singular— bronchus}, their branches, and the lungs. Th e lower tract is also known as the thoracic portion of the respiratory system because Systemic it is enclosed in the thorax. circulation Bones and Muscles of the Thorax CO O2 Exchange III: 2 blood to cells Surround the Lungs CO 2 O2 Th e thorax is bounded by the bones of the spine and rib cage Cellular and their associated muscles. Together the bones and muscles Cells respiration Nutrients are called the thoracic cage . Th e ribs and spine (the chest wall ) ATP form the sides and top of the cage. A dome-shaped sheet of skel- etal muscle, the diaphragm , forms the fl oor ( Fig. 17.2 b). Two sets of intercostal muscles , internal and external, Fig. 17.1 connect the 12 pairs of ribs (Fig. 17.2 a). Additional muscles, the sternocleidomastoids and the scalenes , run from the head and neck to the sternum and fi rst two ribs. dioxide, water, and energy in the form of ATP. External respi- Functionally, the thorax is a sealed container fi lled with three ration, the topic of this chapter and the next, is the movement membranous bags, or sacs. One, the pericardial sac, contains the of gases between the environment and the body’s cells. External heart. Th e other two bags, the pleural sacs , each surround a lung respiration can be subdivided into four integrated processes, il- {pleura, rib or side}. The esophagus and thoracic blood vessels lustrated in Figure 17.1 : and nerves pass between the pleural sacs ( Fig. 17.2 c). 1 Th e exchange of air between the atmosphere and the lungs. Th is process is known as ventilation , or breathing. Inspi- Pleural Sacs Enclose the Lungs ration (inhalation) is the movement of air into the lungs.

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